The present invention pertains to the concentration of slurried particles, for example, concentrating precipitated calcium carbonate slurry from an intermediate solids level to a higher solids level.
In the processing of many materials the material may be dispersed in a slurry which is then subject to further processing to either concentrate the slurried particles or to recover the slurried particles by such conventional techniques as dewatering, filtration and pressing of the dewatered material. A number of industrial processes have attempted to use vacuum columns or vessels to remove the liquid portion, e.g. water from the solid material.
For example U.S. Pat. No. 4,687,546 discloses a method for concentrating kaolin clay using indirect heat exchange to evaporate water from the partially concentrated clay slurry and then vacuum separation to boil off the water at less than normal water boiling temperatures. Patentees require a filtering step to bring the solids concentration to between 50 and 60% prior to heat exchange and flash separation with a starting material being 15% by weight solids.
Patentees in U.S. Pat. No. 5,036,599, cited as an improvement over the previously referred to patent, include a recirculation loop, however, the product was drawn from the recirculation loop and new material for solids concentration is introduced into the separation vessel prior to heat exchange.
U.S. Pat. No. 4,504,355 discloses and claims a vacuum evaporation system for concentrating acetyl cellouse polymer solutions.
U.S. Pat. No. 4,530,737 discloses and claims a process for concentrating a solution by using indirect heat exchange to concentrate black liquor from a sulphate pulping process.
U.S. Pat. No. 5,209,828 discloses vacuum separation of water and volatile organic compounds from a spent caustic stream in a refinery.
The present invention pertains to the concentration of slurried particles for example, concentrating precipitated calcium carbonate slurry from an intermediate solids level to higher solids level.
According to the present invention the slurry to be concentrated is introduced into a vessel, e.g. a vacuum flash column, maintained at a pressure of less than 4 psia, the incoming slurry to be concentrated is heated so that the temperature inside the vacuum flash column is maintained at a temperature below 150xc2x0 F., whereby the water boils off and is removed as steam with other gaseous components. The slurry inside the vacuum flash column is at the concentration desired and is maintained at the concentration by having two recirculating systems that recirculate the slurry. One recirculation system includes means to introduce additional slurry to be concentrated which is subject to external heating in order to maintain the supply temperature. The other recirculation system, at a different location from the first recirculation system, is used for withdrawal of concentrated slurry as product. The method and apparatus of the present invention can be used to raise intermediate solid slurries (e.g. 50 to 60 weight percent) to final solids concentrations of 72 to 76 weight percent or higher with the attendant benefit, and certain possibilities to enhance other properties, such as viscosity and handle ability, when compared to conventional dewatering techniques followed by filtration and press cake dispersion.
Therefore, in one aspect the present invention is a method of concentrating solids in a liquid slurry comprising the steps of: establishing a volume of slurry having a solids concentration lower than a desired higher solids concentration inside a vacuum chamber maintained under a vacuum of less than 4 psia, continuously withdrawing a stream of the slurry from the vacuum chamber and recirculating the stream through a heat exchanger to heat the stream to a temperature sufficient to maintain the temperature of the slurry within the chamber at a minimum temperature less than 150xc2x0 F.; continuously mixing and/or recirculating the slurry contained in the vacuum chamber until a desired higher solids concentration in the slurry is achieved; continuously adding additional slurry having the lower solids concentration, fed into the recirculating stream prior to the recirculating stream entering the heat exchanger; and continuously withdrawing a product slurry having a higher solids concentration than the lower solids concentration fed to the vacuum chamber.
In another aspect the present invention is a system for concentrating solids in a liquid slurry comprising in combination: a vessel adapted to contain a volume of slurry and be maintained under a vacuum of 4 psia or lower; a first recirculation system adapted to continuously withdraw slurry from the chamber and return the slurry into the chamber; heating means in the first recirculation system to heat the recirculating slurry; means to introduce slurry to be concentrated into the first recirculating means; and a second recirculation system at a location away from the first recirculation system to continuously withdraw and recirculate slurry to the vessel, the second recirculation system including means to withdraw concentrated slurry from the second recirculation system.